Both positive and negative pressure mechanical ventilators have been used for decades to treat patients with respiratory disorders. A range of ventilators are described in “Principles & Practice of Mechanical Ventilation”, Edited by M J Tobin (1994, McGrawHill Book Company, ISBN 0-07-064943-X). Other ventilators are described in “Respiratory Therapy Equipment”, by S. P. McPherson (3rd Ed., 1985, C. V. Mosby Company, ISBN 0-8016-3312-5). Other ventilators are described in “Automatic Ventilation of the Lungs” by Mushin et al (3rd Ed, 1980, Blackwell Scientific Publications, ISBN 0-632-002286-7).
Positive pressure ventilators provide a supply of air or breathable gas at positive pressure to a patient's airway. Flow is volume of air per unit time. Tidal volume is the volume of air entering and leaving the lungs during the respiratory cycle. Minute ventilation is the volume of air delivered to a patient in 1 minute. There are two general approaches to control of ventilators: (1) volume or flow; and (2) pressure control. A ventilator may be programmed to control the volume of air delivered to a patient by adjusting the minute ventilation. The rate at which the air is delivered to the patient is breaths (or cycles) per unit time.
In order to achieve the desired minute ventilation, both the rate and volume of air delivered to a patient can be varied.
In this specification, a ventilator will be said to be triggered into an inspiratory phase and cycled into an expiratory phase. Spontaneous breaths are those that are initiated by the patient. If the ventilator determines either the start or end of inspiration, then the breath is considered mandatory. If the patient triggers the ventilator (e.g., with a spontaneous breath), the ventilator is said to be an assistor. If time triggers the ventilator into the inspiratory phase, the ventilator is said to be a controller. If the patient can assist and the machine can back him up (if his breathing rate drops or stops altogether), the ventilator is designated an assistor/controller. It is possible for a machine to be all three.
It is:                (1) an assistor when it is patient-triggered and there is no timed backup rate;        (2) a controller when it is time-triggered and no assist mechanism is provided; or        (3) an assistor/controller when the timed rate backs up the patient's rate (sometimes called “spontaneous/timed”).        
When the ventilator switches between inspiratory and expiratory modes at the same time as the spontaneously breathing patient, the ventilator is said to be in synchrony with the patient. Loss of synchrony can lead to patient discomfort and ineffective ventilation. For purposes of this description, a spontaneous/timed ventilator is considered to be in a spontaneous mode when it is delivering ventilation support in response to detected patient respiration. Similarly, the spontaneous/timed ventilator is considered to be in a timed mode when it is delivering a machine breath according to a back up timing threshold back up rate in response to a failure to detect patient respiration.
A method for providing ventilatory assistance in a spontaneously breathing subject is described in U.S. Pat. No. 6,484,719 (Berthon-Jones), the contents of which are hereby incorporated by cross-reference.
In some situations, a spontaneous/timed ventilator can fail to detect when the patient switches between inspiration and expiration. Therefore some ventilators have a “time-out” for a spontaneous mode. Such ventilators will switch from the spontaneous mode (waiting for the patient) to a timed mode (delivering ventilation at the back up rate} at the end of the time-out period. An improved system for a “time-out” is described in U.S. Pat. No. 6,213,119, the contents of which are hereby incorporated by cross-reference.
As discussed herein, the back-up rate (cycles/time) may be alternatively described by its reciprocal, the back-up period (time/cycle).
In programming an automatic ventilator, the problem arises as to the choice of the most appropriate back-up rate for the device.